1. Field of the Invention
The present invention relates to a rotation member driving device used for an image forming apparatus adopting an image forming system such as an electrophotographic system, an electrostatic recording system, an ionography, or a magnetic recording system and forming a color or black-and-white image, such as a printer, a copying machine, or a facsimile, and an image forming apparatus using the same, and particularly to a rotation member driving device which can rotate and drive a rotation member such as a photosensitive drum or an intermediate transfer drum with high accuracy and can form a high quality image without distortion and out of color registration as a color or black-and-white image, and an image forming apparatus using the same.
2. Description of the Related Art
Conventionally, as this kind of image forming apparatus adopting an electrophotographic system or the like and forming a color or black-and-white image, such as a printer or a copying machine, ones of various kinds of systems have been proposed and have been put on the market. Especially in recent years, as a personal computer, Internet, digital camera, or the like becomes popular, the development of a color printer is remarkable. In the image forming apparatus such as the color printer, an apparatus for office use, which is capable of forming a color image and satisfies a high speed property, compactness and low cost, similar to those for a black-and-white image, is earnestly demanded.
In the image forming apparatus such as the color printer, in order to form a high quality color image, images of respective colors, such as cyan, magenta, yellow and black, are formed on an image holding member, and it is necessary to improve the accuracy of color registration in superposition of these images of the respective colors.
As the image forming apparatus such as the color printer, there are roughly two types, one adopting a system in which only one photosensitive drum as an image holding member is provided, toner images of respective colors, such as cyan, magenta, yellow and black, are sequentially formed on the one photosensitive drum, and the toner images of the respective colors are multiply transferred onto a recording sheet or an intermediate transfer member, so that a color image is formed, and the other adopting the so-called tandem engine in which image forming units each including a photosensitive drum as an image holding member are provided, the number of the units is made to correspond to the number of colors of toner images to be formed, for example, four in the case of cyan, magenta, yellow and black, toner images of respective colors such as cyan, magenta, yellow and black are continuously formed on the photosensitive drums of the respective image forming units, and the toner images of the respective colors are multiply transferred onto a recording sheet or an intermediate transfer member, so that a color image is formed.
Among these, in the image forming apparatus of the system in which only one photosensitive drum is provided, periodic unevenness of speed of a driving device for driving the photosensitive drum or the intermediate transfer member such as an intermediate transfer drum or an intermediate transfer belt causes the accuracy of the color registration to be degraded. In the image forming apparatus of such system, according to the eccentricity of respect shafts of drive transmission parts of the driving device or the dimension accuracy of a tooth profile of a gear or the like, unevenness of rotation occurs in one rotation period of each of the shafts, and in the case where the photosensitive drum or the intermediate transfer member such as the intermediate transfer drum or the intermediate transfer belt is driven through the drive transmission parts of plural stags, there occurs a speed variation in which the unevenness of the rotation in each of the stages is combined. Thus, in the image forming apparatus of the system, setting is made such that the rotation period of the photosensitive drum is integer times as long as the rotation period of the intermediate transfer drum or the intermediate transfer belt, so that the degradation of the accuracy of the color registration due to the periodic unevenness of speed of a driving device is prevented.
In the latter so-called tandem engine image forming apparatus, since plural (for example, four) image forming units for forming images of respective colors are provided, in order to improve the accuracy of the color registration of the images formed in the respective image forming units, it is necessary to make the positions of the images formed by the respective image forming units coincident with predetermined positions with high accuracy. Thus, the tandem engine image forming unit is constructed such that scan start timing in a main scan direction and a sub-scan direction in an image exposure device for exposing an image by a laser beam onto the photosensitive drum, the phase of a polygon mirror for deflecting and scanning the laser beam, the position of a mirror for guiding the laser beam to the photosensitive drum, and the like are controlled, so that the accuracy of the color registration when the images formed by the respective image forming units are overlapped is improved.
However, the above related art has problems as follows. That is, in the image forming apparatus of the system in which only one photosensitive drum is provided, although it is possible to prevent the accuracy of the color registration from degrading to some degree by setting the rotation period of the photosensitive drum and the rotation period of the intermediate transfer drum or the intermediate transfer belt to have the relation of the integer times, since the image forming apparatus of such system is required to sequentially form the toner images of the respective colors, such as cyan, magenta, yellow and black, for every rotation of the photosensitive drum, there is a problem that it is difficult to satisfy the high speed property required for the color printer for office use, or the like.
On the other hand, in the case of the latter image forming apparatus of the so-called tandem engine, since the toner images of the respective colors, such as cyan, magenta, yellow and black, are continuously formed by the plural image forming units, it is sufficiently possible to satisfy the demand for the high speed property. On the contrary, in this kind of tandem image forming apparatus, in order to improve the accuracy of the color registration, since it becomes necessary to perform correction control of controlling the scan start timing of the image exposure device in the main scan direction and the sub-scan direction, the phase of the polygon mirror, and the position of the mirror for guiding the laser beam to the photosensitive drum, there is a problem that it is difficult to reduce the cost.
Further, in the case of the tandem image forming apparatus, even if the problem of the cost resulting from the correction control of the image exposure device in the respective image forming units is solved, as set forth in xe2x80x9cMachine Optimizing Technologies for Color Registration of Electrophotographyxe2x80x9d (Journal of the Imaging Society of Japan, vol. 38, no. 3 (1999) pp. 175-180), since driving portions of photoreceptor shafts of the respective image forming units are constituted by independent drive transmission parts, it is difficult to improve the accuracy of the color registration by phase alignment of unevenness of one rotation period, the position variation generated by the eccentricity or the like is rather large in view of objective color registration accuracy, and the improvement of the accuracy of the driving part becomes a serious problem, which is pointed out in the publication.
Then, in the tandem image forming apparatus constituted by the independent drive transmission parts for the respective image forming units, in order to improve the accuracy of the driving part, various proposals have been made and actually carried out such that the rotation variation of the photosensitive drum of each of the image forming units is detected by an encoder or the like, and the rotation variation of the photosensitive drum detected by this encoder is controlled by feedback control or feedforward control, or an image signal is outputted in a certain period, its image is read on the photosensitive drum or the transfer drum, and a deviation from the output is fed back, so that the accuracy of the color registration is improved.
However, in this case, a detection part and a control circuit for detecting and controlling the rotation variation of the photosensitive drum of each of the image forming units become complicated, and there are new problems that the apparatus is enlarged and the cost is increased, and it is impossible to meet the demand to provide the color image forming apparatus for office use, which satisfies compactness and low cost.
Then, as a technique which can solve such problems, there is one disclosed in Japanese Patent Unexamined Publication No. Hei. 8-194361 or No. Hei. 9-250606.
In an apparatus for preventing out of color registration of color electrophotography according to Japanese Patent Unexamined Publication No. Hei. 8-194361, an image forming system for color electrophotography is constituted by four process units arranged opposite to a passage of a transfer medium, each of these process units includes a photoreceptor and its subsidiary charging device, writing device, development device, transfer device, and cleaning device, and in an image forming apparatus for multiply transferring images formed on the respective photoreceptors sequentially onto the transfer medium, the photoreceptors are constituted by four photoreceptors of a first photosensitive drum, a second photosensitive drum, a third photosensitive drum, and a fourth photosensitive drum in order of transfer to the transfer medium, there are provided a first drum gear, a second drum gear, a third drum gear, and a fourth drum gear each directly fixed to a shaft of each of the photosensitive drums, the first drum gear and the second drum gear are driven by a first drive gear, and similarly, the third drum gear and the fourth drum gear are driven by a second drive gear, and in the case where the distance between the centers of the adjacent photosensitive drums among the respective photosensitive drums is set to equal to the peripheral length of the photosensitive drum early in the transfer order, the first drum gear and the second drum gear are attached to the shafts of the respective photosensitive drums so that respective maximum eccentricity directions of the first drum gear and the second drum gear become the same, and similarly, the third drum gear and the fourth drum gear are attached to the shafts of the respective photosensitive drums so that respective maximum eccentricity directions of the third drum gear and the fourth drum gear become the same as the case of the first drum gear and the second drum gear.
A gear transmission apparatus and an image forming apparatus of Japanese Patent Unexamined Publication No. Hei. 9-250606 are constructed such that a first gear connected to a drive source side, a second gear connected to a side of an object to be driven, a third gear engaged with the first gear, and a fourth gear attached on the same axis as the third gear and engaged with the second gear are provided, one of the third gear and the fourth gear has the number of teeth an odd number of times as large as the other, and the timing when the variation of rotation speed of the gear by engagement between the first gear and the third gear becomes maximum is almost coincident with the timing when the variation of rotation speed of the gear by engagement between the second gear and the fourth gear becomes maximum.
In the case of the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-194361 or No. Hei. 9-250606, the construction is such that the first drum gear and the second drum gear are attached to the shafts of the respective photosensitive drums so that the respective maximum eccentricity directions of the first drum gear and the second drum gear become the same, and similarly, the third drum gear and the fourth drum gear are attached to the shafts of the respective photosensitive drums so that the respective maximum eccentricity directions of the third drum gear and the fourth drum gear become the same as the case of the first drum gear and the second drum gear, or the construction is such that the timing when the variation of the rotation speed of the gear by the engagement between the first gear and the third gear becomes maximum is almost coincident with the timing when the variation of the rotation speed of the gear by the engagement between the second gear and the fourth gear becomes maximum. However, in the case of these techniques, although the phases of the speed variations of the respective photosensitive drums due to the eccentricity of the drum gears or the like can be aligned with each other to some degree, it is impossible to decrease the speed variation generated during one rotation of the photosensitive drum, and they still have a problem that the accuracy of the color registration can not be sufficiently improved. Besides, although there is a disclosure that the timing when the variation of the rotation speed of the gear by the engagement between the first gear and the third gear becomes maximum, its specific method is not disclosed.
The present invention has been made in view of the above problems and provides a rotation member driving device in which without enlarging the size of the device and without increasing the cost, a speed variation generated during one rotation of a rotation member such as an image holding member or an intermediate transfer member is reduced, so that the distortion and out of color registration of an image formed on or transferred onto the rotation member such as the image holding member or the intermediate transfer member is reduced, and a high quality image can be formed, and an image forming apparatus using the same.
According an aspect of the present invention, the rotation member driving device has a rotation member for forming an image, having an eccentricity, and a gear attached to the rotation member for rotating thereof. In the device, a relation between a phase of the eccentricity of the rotation member and a phase of a cumulative pitch error in a direction of rotation of the gear is set so that a variation of a surface speed of the rotation member due to the eccentricity thereof is restrained from appearing on an image to be formed.
When a position where the eccentricity of the rotation member is relatively large is moved to an image-forming position, the cumulative pitch error of the gear in the rotation direction may become relatively small at an engagement position of the gear.
Furthermore, when neighborhood of a phase where the eccentricity of the rotation member is approximately maximum is moved to an image-forming position, the cumulative pitch error of the gear in the direction of rotation may become approximately minimum at an engaging position of the gear.
In the device, marks may be respectively provided at a position where the eccentricity of the rotation member becomes maximum and a position where the cumulative pitch error of the gear in the rotation direction becomes minimum, and the gear may be attached to the rotation member in accordance with the marks.
The image-forming position of the rotation member and the engagement position of the gear may be set to be substantially opposite positions or substantially same positions.
According to another aspect of the present invention, the image forming apparatus has an image holding member having an eccentricity and a gear attached to the image holding member for rotating thereof. When a position of the image holding member where the eccentricity is relatively large is moved to a position where exposure is performed, a cumulative pitch error of the gear in a direction of rotation becomes relatively small at an engaging position thereof.
According to another aspect of the present invention, the image forming apparatus has an image holding member that holds an image, at least one intermediate transfer member having an eccentricity, onto which the image is transferred, and a gear attached to the intermediate transfer member for rotating thereof. When a position of the at least one intermediate transfer member where the eccentricity is relatively large is moved to an image transfer position, a cumulative pitch error of the gear in a direction of rotation becomes relatively small at an engaging position thereof.
According to another aspect of the present invention, the image forming apparatus forms a color image by forming latent images respectively according to pieces of input information of plural colors, developing the latent images with toners of corresponding colors to obtain plural single-color toner images, and fixing the plural single-color toner images on a recording medium. The image forming apparatus has at least three image holding members on which latent images respectively corresponding to pieces of input information of plural colors are formed, and the latent images are developed with toners of corresponding colors to form plural single-color toner images, a gear attached to each of the at least three image holding members for rotating thereof, at least one intermediate transfer member which is disposed in contact with or close to the image holding member and to which the single-color toner images formed on the image holding members are transferred, and a final transfer rotation member for transferring the toner images transferred onto the at least one intermediate transfer member to a recording medium. At least two image transferring cycles are performed, and when a position of the image holding member where eccentricity is relatively large is moved to a position where exposure is performed, a cumulative pitch error of the gear in a rotation direction becomes relatively small at an engagement position of the gear.
According to another aspect of the present invention, the image forming apparatus also forms a color image by forming latent images respectively according to pieces of input information of plural colors, developing the latent images with toners of corresponding colors to obtain plural single-color toner images, and fixing the plural single-color toner images on a recording medium. The apparatus has a single image holding member on which latent images respectively corresponding to pieces of input information of plural colors are sequentially formed and the latent images are developed with toners of corresponding colors to form plural single-color toner images sequentially, a gear attached to the image holding member for rotating thereof, at least one intermediate transfer member which is disposed in contact with or close to the image holding member and to which the single-color toner images formed on the image holding member are transferred, and a final transfer rotation member for transferring the toner images transferred onto the at least one intermediate transfer member to a recording medium. At least two image transferring cycles are performed, and when a position of the image holding member where eccentricity is relatively large is moved to a position where exposure is performed, a cumulative pitch error of the gear in a rotation direction becomes relatively small at an engagement position of the gear.